Final answer:
Professor Martin is describing the approach called Systems theory, which emphasizes understanding psychological problems within the context of a dynamic system where each member plays a unique role.
Step-by-step explanation:
Professor Martin's description of investigating interactions among relatives or within the context of a dynamic system in psychotherapy refers to the approach called Systems theory. This approach focuses on the complex interactions and relationships within a system, such as a family, and how each member's unique role contributes to psychological problems. Systems theory recognizes that individual behavior and psychological issues cannot be fully understood without considering the larger context in which they occur.